Linear blending?

[virtualritz]

One of the things Skia gets right is that it uses linear color for all blending operations (as an aside it's also the only OSS vector graphics library I'm aware of that has proper color space support). The linear stuff is pretty important for anything that involves transparency. E.g. antialiasing. Aka: all colors get converted to linear before blended in any way and then converted back to whatever space they were in before, for display.

I just ran the stroke example and peeking at the edges of the lines I see the darkening in the semi-transparent pixels that is typical for blending wrongly in a gamma-corrected color space. It's easy to spot on the blue set of lines.

Of course I may be wrong and this artifact is coming from elsewhere.

[RazrFalcon]

Right now, tiny-skia supports only sRGB. I do plan to add LinearRGB eventually, but not in the first release.

[virtualritz]

Hmm, can you say what 'supports' means? Specifying colors in sRGB but they still get blended linearly, internally? That would be enough. I.e. I am not asking for specifying colors in linear quantities. I'm just asking if tiny-skia is correctly blending them internally.

[RazrFalcon]

Currently, tiny-skia works exactly the same as the Skia itself. So unless there is a bug that I've missed, this is the expected result.

Skia supports linear color space as an option, which is not enabled by default. And this is how tiny-skia works.

[RazrFalcon]

I've just recreated the stroke example in Skia and it produces exactly the same result. Pixel-perfect. So it's not a bug.

[virtualritz]

Maybe it's something else. I just realized the stroke example has no background. So what I'm seeing is the Preview utility of macOS doing alpha compositing. Maybe what is missing/wrong is the color encoding in the PNG that the stroke example writes.

Heaps sorry about the noise. I will investigate and re-open, if necessary.

[RazrFalcon]

No problem. The library is still in an alpha stage, so anything can happen.

[virtualritz]

The library is still in an alpha stage, so anything can happen.

Hehe, I'm using for drawing UI elements for a real product. I'll post screenshots soon.

[virtualritz]

Btw, I was referring to this. Apparently you can use Skia w/o correct color so that may or may not explain why your result matches Skia's.

[RazrFalcon]

Yes, color spaces are not supported and it's a very complex task. I do plan implementing LinearRGB, but I'm not sure about ICC. It's just too much work.

[virtualritz]

You only need to have a way to annotate any bitmap with a color space and single colors, e.g. for strokes, fills or ramps, you can store linear right away. Maybe use palette to do the heavy lifting for all that?

The important bit is that premultiplication/unpremutiplication/filtering/blending should only ever happen in linear. And for that you need at least 16bit precision per component (u16/f16(half)) but probably fastest on most architectures is just f32. That is pretty much what you wrote: linear RGB. That's all you ever need to do.

I'm happy to help out if there are questions.

[RazrFalcon]

Yes, I know how to implement this. But it would not be available in v0.2, i.e. anytime soon.

[virtualritz]

Hehe, I'm using for drawing UI elements for a real product. I'll post screenshots soon.

Screen shot from an AfterEffects plug-in I'm working on. The Envelope widget is drawn withtiny-skia. All other widgets are host app native btw.:

[RazrFalcon]

Nice!